146 HUMAN ANATOMY. 



occipito-atloid joint and 'no' at the atlo-axoid" (Owen), Painless nodding and 

 rotation of the head aid, therefore, in the exclusion of the occipito-atlantal and 

 atlanto-axial regions in obscure cases of high caries. 



The axis is more spongy than the atlas, and is weakest about one centimetre 

 below the neck of the odontoid process, and this is one of the most frequent seats 

 of fracture. 



In fracture-dislocations, which constitute from seventy to eighty per cent, of se- 

 vere spinal injuries, the thoracico-lumbar region suffers most commonly for the reasons 

 above stated. The almost vertical direction of the articular processes of the thoracic 

 vertebrae causes them, when flexion is extreme, as when a weight has fallen on the 

 back, to be frequently fractured, which, together with the accompanying crushing 

 of the vertebral body and rupture of the supra- and interspinous ligaments and 

 the ligamenta subflava, permits the immediate sliding forward of the vertebrae above 

 the crushed one and the compression of the cord — often its practical severance — • 

 between the anterior edge of the posterior arch of the upper vertebra and the poste- 

 rior edge of the body of the lower one, 



(For the resulting symptoms, see section on Nervous System, page 1053.) It 

 may be mentioned here that the spinal nerves do not arise from the cord opposite 

 the vertebrae after which they are named. Th^ir regions of origin may briefly be 

 stated as follows : 



(i) Occiput to sixth cervical spine, — eight cervical nerves. 



(2) Seventh cervical to fourth thoracic spine, — upper six thoracic nerves. 



(3) Fifth to tenth thoracic spine, — lower six thoracic nerves. 



(4) Eleventh and twelfth thoracic spines, — five lumbar nerves. 



(5) First lumbar spine, — five sacral nerves. 



Landmarks. — To fix the limits of the spine in the living, draw a horizontal 

 line from the anterior nasal spine to the lower edge of the external occipital pro- 

 tuberance and another backward from the top of the symphysis pubis. Seen from 

 the side, the top of the spine is in a line connecting the front of the lobe of each ear, 

 passing behind the neck of the lower jaw. Frozen sections show that the front of 

 the vertebral bodies is much nearer the centre of the body than one is prepared to 

 expect. A vertical transverse, or frontal, plane through the thorax at its greatest 

 breadth strikes the angle of the jaw, the front of the cervical convexity of the spine, 

 and cuts the body of the fourth lumbar (Langer). 



The relations of the spine anteriorly are considered with the parts in front of it. 

 The parts felt from the surface are the spinous processes and some few of the trans- 

 verse ones. The line of the spines is a good example of the general rule that 

 prominences on the skeleton lie in hollows in the flesh ; a deep furrow between the 

 muscular masses marks their position. 



Palpation of the normal spine with the soft parts in place gives the following 

 information. The spine of the second cervical can be felt by deep pressure a little 

 below the occiput. The short spines of the succeeding vertebrae are made out with 

 great difficulty. The fifth is longer than those just above it. The sixth is much 

 longer and nearly as long as that of the seventh. The name vertebra prominens 

 conferred on the seventh is misleading, for the spine of the first thoracic is the most 

 prominent in this region. The third, fourth, and fifth cervical spines recede from 

 the surface by reason of the forward curve of the cervical segment and on account 

 of their shortness. This permits of free extension of the head and neck. The liga- 

 mentum nuchae also prevents them from being felt distinctly. The sixth and seventh 

 cervical and first thoracic are easily felt. The remainder, lying in the groove caused 

 by the prominence of the erector spinae muscles, can usually be palpated without 

 much difficulty 



The relative sizes vary so much that it is not safe to identify any spine in this 

 way. If the whole series from the second cannot be counted, it is best to start from 

 the fourth lumbar, which is on a level with the highest points of the ilia. Vertebrae 

 can also be identified from the lower ribs by the relations of the heads to the bodies. 

 The relations of the spinous processes to the' body vary. Thus, in the cervical 

 region the first five spines pass nearly straight backward. The sixth and seventh, 

 like the upper two or three thoracic spines, descend a little, so that the tip is opposite 



